In U.S. Pat. No. 2,672,677 there has been described a descaling tool wherein a cluster of pins project from an open end of a tubular tool housing and are axially reciprocable, by pneumatic or other means, within a metallic guide plate which is limitedly displaceable in the direction of pin motion. The pins slide in respective bores of the guide plate and are rearwardly terminated by heads larger than these bores which are urged against the rear face of that plate by a resilient cushion, inserted between the guide plate and a reciprocating hammer, or by biasing springs individually bearing upon shoulders of the pins lying forwardly of the guide plate.
The high operating frequency of such a tool generates frictional and other forces which on the one hand lead to an early destruction of the pins and on the other hand cause a gradual widening of the bores of the guide plate. Thus, the needles were found to break at their heads within a few tens of hours of operation.